Did you read through the other sound threads around?
http://www.captainsim.org/yabb2/YaBB.pl?num=1286037561

Here's a quote with a thing you haven't mentioned yet.

Quote:

Copy the msvcr71.dll file C:\Program Files\Windows\system32 folder and paste it into [Microsoft Flight Simulator X Root Folder]. If you can't find the file, go to Microsoft Download Center to get it.

Oops, I just saw that this tip is also mentioned in this thread on page one. But maybe you haven't tried it yet.